1. Field of the Invention
The present invention relates to an image-shake correcting system for correcting an image shake due to a vibration of a camera which occurred during photography, during a reproducing operation of a video reproducing apparatus, during a receiving, operation of a communication receiver, or the like.
2. Description of the Related Art
The image-shake correcting system shown in FIG. 1 is described in Japanese Laid-Open Patent Application No. Sho 63-166370 as one example of an image-shake correcting system suitable for use in a video reproducing apparatus having the function of correcting an image shake due to a vibration of a camera which occurred during photography.
Referring to FIG. 1, a video reproducing apparatus 1 is provided for reproducing a video signal and outputting a reproduced video signal P1. An image memory 2 is provided for temporarily storing the input video signal P1. A motion-vector detecting circuit 3 is provided for detecting a motion vector from the input video signal P1 and outputting a motion vector P2 and a maximum value P3 of a correlation coefficient of the previous image and the current image. A microcomputer 4 is provided for controlling the image memory 2 and an image enlarging circuit 5, and outputs a reading address P4 and digital data P5 indicative of an image enlargement ratio. The image enlarging circuit 5 is provided for enlarging an image read from the image memory 2 up to a normal image-plane size and providing an output video signal P6. Specifically, the motion of the image is detected from a displacement between a plurality of images stored in the image memory 2, and the area of the image memory 2 from which an image is to be read is shifted in a direction in which the motion can be cancelled. Accordingly, the size of the read image is smaller than the normal image-plane size, and the read image is enlarged up to the normal image-plane size.
In the image-shake correcting system arranged in the above-described manner, the video signal P1 reproduced by the video reproducing apparatus 1 is temporarily stored in the image memory 2 and is also supplied to the motion-vector detecting circuit 3. The motion-vector detecting circuit 3 detects the motion vector P2 indicative of the moving speed of the image from the input video signal P1 and supplies the motion vector P2 to the microcomputer 4. The microcomputer 4 determines the reading address P4 of the image memory 2 on the basis of the information indicated by the motion vector P2, and reads image information indicative of an image smaller than the original image from the image memory 2 while shifting this smaller image horizontally or vertically.
The image information read from the image memory 2 is supplied to the image enlarging circuit 5, and the read image is corrected in accordance with the digital data (image enlargement ratio) P5 supplied from the microcomputer 4, so that the read image is enlarged up to the normal image-plane size.
If an image shake exists in the reproduced video signal P1, it is corrected through the above-described series of operation, so that the video signal P6 indicative of a stable image free of the image shake is outputted.
A most general method for finding the motion vector P2 is a matching method. The matching method includes the steps of finding a correlation coefficient while displacing a recognition pattern of an immediately previous field and a recognition pattern of the current field with respect to each other, and determining as a motion vector the amount of displacement obtained when the correlation coefficient is a maximum. The motion-vector detecting circuit 3 is arranged to perform a calculation on the motion vector P2 in this manner, and outputs both the motion vector P2 and the maximum value P3 of the correlation coefficient to the microcomputer 4. The maximum value P3 of the correlation coefficient is used for evaluating the reliability of the motion vector P2.
There is another method of dividing an image plane into a plurality of area blocks and detecting a motion vector for each of the area blocks for the purpose of deforming a subject in the image plane, eliminating a subject which cannot be processed or automatically determining a subject the image of which is to be stabilized. In such a method, the microcomputer 4 combines the motion vectors obtained from the respective area blocks and calculates the amount of vibration of the entire image plane, by means of averaging or median processing. Since the thus-calculated value indicates a difference between frames or fields with reference to the position of an image, integration or filtering processing is performed to determine a final image correction vector, i.e., the reading address P4 of the image memory 2. Incidentally, the image enlargement ratio P5 is normally set to a constant value for each photographic state.
However, in the above-described conventional example, if image-shake correction is applied to an actually broadcast television image or an image recorded by a domestic-use video camera, the following problems take place. For example, in such a broadcast or recorded image, scene changes frequently occur (an image changes into an utterly different image). Particularly in the case of the image recorded by a domestic-use video camera, noise or synchronization deviation may occur each time a scene change appears. If the video signal P1 containing the noise or synchronization deviation is directly inputted to the motion-vector detecting circuit 3, no correlation between images is obtained and, each time a scene change appears, a motion-vector detection error occurs. If image-shake correction is simply applied to such a video signal P1, the image will instantaneously vibrate to a great extent and will be displayed as a disagreeable image.
Also, if an image is cut out from a peripheral portion of the original image when the reading address P4 is close to a limit of an address control range immediately before a scene change, the reading address P4 reaches its control limit value beyond which no image-shake correction can be performed, due to the motion-vector detection error during the scene change. In this case, the image-shake correction is impossible for a while after the scene change.